1. Field of the Invention
The invention relates to an apparatus and process for the indirect heat drying and simultaneous pelletization of dewatered sludge, such as sludge obtained from a wastewater treatment process. The mechanically dewatered sludge is thermally dried and processed to obtain sterile pellets, uniform in particle size and moisture content. Thereafter, the pellets are suitable for use as a fertilizer, an additive to a fertilizer, or as a fuel.
2. Description of Related Art
Municipal sludge is mostly a liquid containing 2 to 6% total solids. It typically contains inorganic and organic matter, nutrients such as nitrogen, phosphorous and potassium and traces of various metals. It may also contain pathogens, and, in some instances, constituents such as heavy metals and hazardous organics, depending upon the source of the raw sewage that has been treated.
In the known methods of treating municipal sludge, the sludge is treated to increase its solid content. When liquid sludge is mechanically dewatered, the dewatered sludge solids content is usually in the 15 to 30% of total solids. When the dewatered sludge is thermally dried, a product of 90 to 95% total solids is obtained.
The thermal treatment of sludge has the advantage of the greatest reduction in sludge volume (up to 98%), and also destroys or inactivates pathogenic organisms, rendering the sludge sterile. On the other hand, the thermal treatment of sludge requires special drying equipment and an energy source for generating the heat needed in drying the sludge.
Two types of sludge thermal drying are known in the treatment of municipal sludge. In the first type of system, known as direct drying, hot gas is brought into direct contact and mixed with the sludge in a chamber to vaporize water in the sludge directly. The problem with direct drying of sludge is that a large volume of malodorous gas is generated in the drying process that subsequently needs to be treated in order to remove particulate matter and to maintain an odorless treatment process.
Another known type of thermal treatment of municipal sludge uses an indirect sludge drying process wherein the sludge contacts a heated surface to vaporize the water contained therein. One known type of an indirect dryer is illustrated schematically in FIG. 2. The sludge is dried by contacting metal surfaces heated by a liquid or gaseous heating medium, such as steam, hot gas, thermal oil, etc. so that direct contact between the heating medium and the sludge is prevented. Thus, the heating medium remains clean and uncontaminated. The dryer exhaust containing water vapor and air (sweep air) which is introduced into the indirect dryer in limited quantity to facilitate transport of the water vapors is of relatively low volume. The organic matter and nutrients remain in the dried sludge product that exits the dryer. Further, the sludge is exposed to a temperature of above 100.degree. C. for a sufficient time to inactivate or destroy any living organisms contained therein such as viruses, pathogens, etc.